When, roughly, do you think the source of the Amazon was discovered? A pretty safe guess might seem to be in the mid-19th century, around the same time as the Nile’s. But it would also be hopelessly wrong - because the discovery was made by a man called Loren McIntyre in 1971.

The reason it took so long gradually becomes apparent in Petru Popescu’s eye-popping account of McIntyre’s Amazonian adventures. But these adventures are so extraordinary that the mere clearing up of a centuries-old mystery is by no means the most thrilling story in the book.

Take the first part, set in 1969 - but also, it soon turns out, in the Stone Age.

Loren McIntyre discovered the source of the Amazon river in 1971. The photographer flew deep into the jungle to find the remote Mayoruna tribe, who believe they are descended from jaguars, above

American magazine photographer McIntyre flew deep into the jungle on a quest to find the remote Mayoruna tribe, who believe they’re descended from jaguars - and to prove it, pierce their lips and cheeks with thin wooden spikes in a passable imitation of whiskers. (Hence their nickname, ‘the cat people’.)

As well as the pilot, McIntyre had with him an Indian guide. But shortly after landing, the guide developed malaria, so the pilot agreed to take him back and return two days later to meet McIntyre in the same spot.

The plan spectacularly failed. Early the next morning, McIntyre was approached by four whiskered hunters carrying dead monkeys and wearing the traditional Mayoruna garb of nothing at all.

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After some wordless negotiations involving such time-honoured explorers’ items as mirrors and trinkets, he was allowed to follow them with his camera.

Several hours of twisting jungle paths later, he realised he had no idea how to get back to his base. ‘I’m either in a jam,’ he decided, ‘or starting a great experience.’

In the event, it was both. The hunters eventually led him to a makeshift village where, unable to speak the language and separated from the locals by ‘a psychological distance of 20,000 years’, he now felt ‘alone and lost in prehistory’.

Nor could he work out why the Mayoruna were burning their most precious artefacts and constantly moving from one site to another.

What did seem clear, however, was that his presence had divided the tribe. The good news was that the headman (whom McIntyre nicknamed Barnacle, because of his many warts) appeared to be on his side. The bad news was that one of the hunters he’d first met (whom, less imaginatively, he nicknamed Red Cheeks because of his red cheeks) certainly wasn’t.

McIntyre was having trouble holding on to his own beliefs - and even his own identity

And just to remove any doubt, one night Red Cheeks led him far out of the village on a pretend torch-lit hunt, pushed him into a thorn bush and left him to die.

Fortunately, a couple of days later, while digging out a wide selection of flesh-eating insects from his body with a penknife, McIntyre was rescued by Barnacle’s men.

On his return to the village, he found Red Cheeks’s corpse prominently displayed to show any other potential dissenters that the coup had failed.

And after that, things took a turn for the strange. Whenever he sat near Barnacle, McIntyre could ‘hear’ the man talking to him telepathically.

In this way, he learned that the Mayoruna were doing all that decluttering so as to return to the beginning of time when life was simpler and the world less scary. (The Amazonian equivalent, I would suggest, of the hippies’ desire at Woodstock to get back to the Garden of Eden.) Stranger still, when McIntyre finally met a Mayoruna who could speak Portuguese, the man confirmed everything that he’d ‘heard’ from Barnacle about ‘the return to the beginning’. The same man also explained that telepathic communication - the ‘beaming’ of the book’s subtitle - was something that all self-respecting Mayoruna elders could do.

By this time, not surprisingly, McIntyre was having trouble holding on to his own beliefs - and even his own identity. The Mayoruna couldn’t understand, let alone accept, the ability of white people to behave so differently from those around them, as if they had no common goal.

Didn’t they realise there’s no room for individuality when all that matters is the survival of the tribe?

The Encounter: Amazon Beaming by Petru Popescu (Pushkin Press £9.99)

Nonetheless, it was with some regret and guilt - as well as relief - that McIntyre finally managed to escape when a flash flood handily swept through the Mayoruna’s latest village. Amid the debris, it brought was a balsa wood raft that he clung on to for several days as he floated back to the 20th century.

Yet, once he got there, it was Barnacle’s influence (and occasionally, voice) that made him decide to make his own more literal return to the beginning, by finding once and for all the Amazon’s much-disputed source.

In most books, the subsequent discovery would be a guaranteed big finish. Here, after all that’s led up to it, it proves, if anything, a bit of an anti-climax.

Basically, McIntyre and two colleagues headed for the Andes where, give or take the odd minor earthquake and near-death experience caused by high-altitude sickness, they found the source without much trouble. (For the record, it’s a lake now known as Laguna McIntyre.)

But it’s also in this final section that another of the book’s more curious aspects comes to the fore.

In Woody Allen’s film Radio Days, there’s a married couple so argumentative that they can fight bitterly about whether the Pacific or the Atlantic is the greater ocean.

And underlying The Encounter, it transpires, is a similarly rancorous dispute about the world’s greatest river. ‘The Nilists’, as Popescu scornfully calls them, may throw their weight about in geographical circles. Yet not only is the Nile definitely shorter than the Amazon (whatever its deluded champions may claim), but its source was an utter doddle to discover.

After all, Africa’s ‘vast open spaces easily allowed the drawing of detailed maps’ - whereas ‘Amazonia, impossibly unyielding, only gained its first complete maps with the advent of aerial photography’.

The Nile also ‘flows . . . mostly along a single channel for the final 1,200 miles’ through comparatively friendly terrain, making the task of marching along it almost childishly simple.

The Encounter, first published in 1991, is being reprinted now because Simon McBurney (best known as the sinister Archdeacon in the TV series Rev) has adapted it into an acclaimed one-man show.

According to several excited critics, McBurney’s use of the very latest in sound technology has the miraculous effect of making you feel you’re right there in the Amazon jungle with Loren McIntyre and the Mayoruna.

But for proof that the same effect can be produced by the rather less advanced method of ink on paper, look no further than this book.